6-0000 STORM DRAINAGE TABLE OF CONTENTS 6-0100 GENERAL INFORMATION See current PFM ..................................................................................... 5 6-0200 POLICY AND REQUIREMENTS FOR ADEQUATE DRAINAGE ................................................ 5 6-0201 Policy of Adequate Drainage See current PFM .......................................................................................... 5 6-0202 Minimum Requirements See current PFM ................................................................................................. 5 6-0203 Analysis of Downstream Drainage System .................................................................................................. 5 6-0204 Submission of Narrative Description and Downstream Analysis ............................................................... 9 6-0205 Small Private Drainage System See current PFM .................................................................................... 10 6-0300 POLICY ON DETENTION OF STORMWATERS ........................................................................... 11 6-0301 General Policy ............................................................................................................................................... 11 6-0302 Detention Measures ...................................................................................................................................... 11 6-0303 Location of Detention Facilities .................................................................................................................. 13 6-0400 STORMWATER RUNOFF QUALITY CONTROL CRITERIA .................................................... 15 6-0401 General Information and Regulations ......................................................................................................... 15 6-0402 Stormwater Quality Control Practices ......................................................................................................... 17 6-0500 POLICY ON OFF-SITE DRAINAGE IMPROVEMENTS See current PFM ............................... 21 6-0600 POLICY ON PROPORTIONATE COST OF OFF-SITE DRAINAGE IMPROVEMENTS See current PFM ............................................................................................................................................................... 21 6-0700 POLICY ON WHAT MAY BE DONE IN FLOODPLAINS See current PFM .............................. 21 6-0800 HYDROLOGIC DESIGN ........................................................................................................................ 22 6-0801 Acceptable Hydrologies See current PFM ................................................................................................ 22 6-0802 NRCS Hydrology ......................................................................................................................................... 22 6-0803 Rational Formula .......................................................................................................................................... 22 6-0804 Anderson Formula See current PFM ......................................................................................................... 26 6-0805 Other Hydrologies See current PFM ......................................................................................................... 26 6-0806 Incremental Unit Hydrograph – 1 Impervious Acre See current PFM ................................................... 26 6-0900 CLOSED CONDUIT SYSTEM See current PFM ................................................................................ 27 6-1000 OPEN CHANNELS See current PFM .................................................................................................... 27 _____________________________________________________________________________________________ 2011 PFM for Grandfathered and Time Limited Projects Page 6-1 6-0000 STORM DRAINAGE 6-1100 STORM SEWER APPURTENANCES See current PFM .................................................................. 27 6-1200 CULVERTS See current PFM .................................................................................................................. 27 6-1300 RETENTION, DETENTION, AND LOW IMPACT DEVELOPMENT FACILITIES .............. 28 6-1301 General Requirements .................................................................................................................................. 28 6-1302 Rooftop Storage ............................................................................................................................................ 29 6-1303 Percolation Trenches .................................................................................................................................... 33 6-1304 Pervious Pavement ....................................................................................................................................... 36 6-1305 Retention and Detention Ponds ................................................................................................................... 54 6-1306 Maintenance Design Considerations ........................................................................................................... 62 6-1307 Bioretention Filters and Basins .................................................................................................................... 64 6-1308 Vegetated Swales .......................................................................................................................................... 82 6-1309 Tree Box Filters ............................................................................................................................................ 97 6-1310 Vegetated Roofs .......................................................................................................................................... 103 6-1311 Reforestation ............................................................................................................................................... 111 6-1400 FLOODPLAIN See current PFM .......................................................................................................... 116 6-1500 ON-SITE MAJOR STORM DRAINAGE SYSTEM See current PFM ......................................... 116 6-1600 DESIGN AND CONSTRUCTION OF DAMS AND IMPOUNDMENTS See current PFM ..... 116 6-1700 POLICY ON WHAT MAY BE DONE IN CHESAPEAKE BAY PRESERVATION AREAS See current PFM ....................................................................................................................................................... 116 _____________________________________________________________________________________________ 2011 PFM for Grandfathered and Time Limited Projects Page 6-2 6-0000 STORM DRAINAGE LIST OF TABLES Table 6.3 Phosphorus Removal Efficiencies ........................................................................................................... 17 Table 6.5 Runoff Coefficients and Inlet Times ....................................................................................................... 25 Table 6.20 Mass Diagram Analysis ......................................................................................................................... 35 Table 6.21 Aggregate Gradation .............................................................................................................................. 45 Table 6.22 Filter Fabric Specifications .................................................................................................................... 46 Table 6.23 2-Year Storm Routing ............................................................................................................................ 57 Table 6.24 10-Year Storm Routing .......................................................................................................................... 61 Table 6.25 Pretreatment Filter Strip Sizing ............................................................................................................. 67 Table 6.26 Pretreatment Vegetated Channel Sizing ................................................................................................ 67 Table 6.27 Compost Specifications .......................................................................................................................... 73 Table 6.28 Soil Media Specifications ...................................................................................................................... 73 Table 6.29 Filter Fabric Specifications .................................................................................................................... 73 Table 6.30 Channel Segment Length ....................................................................................................................... 86 Table 6.31 Channel Cross-section Area ................................................................................................................... 87 Table 6.32 Example Problem ................................................................................................................................... 93 Table 6.33 Granular Drainage Media Specifications ............................................................................................ 106 Table 6.34 Filter Fabric Specifications .................................................................................................................. 106 Table 6.35 Growth Media Specifications .............................................................................................................